A templating approach for monodisperse self-assembled organic nanostructures

Article abstract of DOI:10.1021/ja710749q

The precise structural control is known for self-assembly into closed spherical structures (e.g., micelles), but similar control of open structures is much more challenging. Inspired by natural tobacco mosaic virus, we present the use of a rigid-rod template to control the size of a one-dimensional self-assembly. We believe that this strategy is novel for organic self-assembly and should provide a general approach to controlling size and dimension. Copyright

Full text of DOI:10.1021/ja710749q

Published on Web 02/12/2008
A Templating Approach for Monodisperse Self-Assembled Organic
Nanostructures
Steve R. Bull,† Liam C. Palmer,† Nathaniel J. Fry,† Megan A. Greenfield,‡ Benjamin W. Messmore,†
Thomas J. Meade,†,§ and Samuel I. Stupp*,†,¶
Departments of Chemistry, Biochemistry and Molecular and Cellular Biology, Neurobiology and Physiology, and
Radiology, and Departments of Chemical & Biological Engineering, Materials Science & Engineering, and
Feinberg School of Medicine, Northwestern UniVersity, EVanston, Illinois 60208
Received November 30, 2007; E-mail: s-stupp@northwestern.edu
The noncovalent self-assembly of molecules into supramolecular
nanostructures presents exciting opportunities for complex structure
and function inaccessible through step-by-step synthesis.1 In systems
that generate supramolecular objects on the nano- and micrometer
scales, it is desirable to control precisely nanostructure size in all
dimensions.2 While precise structural control is known for closed
spherical structures (e.g., micelles), the control of open structures
is much more challenging. The tobacco mosaic virus (TMV)
assembly represents an excellent example of templating by mac-
romolecules. In that system, more than 2100 coat proteins spontane-
ously assemble into a 300-nm-long, rod-shaped structure around a
single RNA molecule. The length of the assembly is determined
Figure 1. (a) Peptide amphiphile monomer 1 and a cartoon representation.
(b) Oligo(phenylene ethynylene) dumbbell 2 (R ) 2-octyldodecyl, PEG )
(OCH2CH2)nOCH3, where n ) 7.4) and cartoon representation. (c,d) Cartoon
by the length of the enclosed viral RNA template.3 The TMV coat
proteins also self-assemble without RNA, affording structures
that have the same diameter as native (untemplated) capsids but
representation of the PA self-assembling around the dumbbell molecule
vary in length. The RNA template inhibits the unlimited self-
assembly of the caspid proteins through specific interactions
between constituent molecules. Herein, we describe an analogous
strategy to control the length of a self-assembled nanofiber by using
a rigid template.
creating a structure of discrete length.
The self-assembling molecule used in this study is peptide
amphiphile (PA) 1, which consists of a charged amino acid sequence
covalently attached to a hydrophobic alkyl chain (Figure 1).4 These
systems typically self-assemble into nanofibers as a result of
hydrophobic collapse of the alkyl chains and â-sheet formation by
the peptide segments. As in the TMV system without a template,
PA monomers self-assemble into high-aspect-ratio nanofibers with
uncontrollable lengths (micrometer scale) and well-defined diam-
eters unless extensive bundling occurs among nanofibers. The
average diameter of nanofibers formed by 1 was measured by
atomic force microscopy (AFM) to be 5.3 ( 0.6 nm (Figure 2a)
and is in good agreement with the width determined by transmission
electron microscopy (TEM).
We designed and synthesized template molecule 2 that contains
a hydrophobic oligo(p-phenylene ethynylene) core with bulky
hydrophilic end caps in the shape of a dumbbell.5-7 The hydro-
phobic rigid-rod defines a precise length that can be encapsulated
within the hydrophobic core of the PA nanofibers. We hypothesized
that in an aqueous solution, PA 1 and template 2 would co-aggregate
through hydrophobic collapse.8 We also assume that the hydrophilic
polyethyleneglycol (PEG) caps at each end of the rigid rod would
disrupt the one-dimensional fiber assembly (Figure 1c,d).
When 2 alone was drop-cast from aqueous solution on mica,
AFM showed ribbon-like supramolecular structures (see Supporting
Information). We believe that these dumbbell molecules aggregate
upon drying on the hydrophilic substrate, maximizing π-π stacking
Figure 2. AFM height images of aqueous solutions drop-cast on mica (1
mg/mL of 1). (a) Peptide amphiphile 1 alone gave fibers with heights of
5.3 ( 0.6 nm. (b) A 200:1 molar ratio mixture of 1 and 2 showed no fibers
in a wide-field view.
interactions between rigid-rods. Similar self-assembly has been
reported previously with a related oligo(p-phenylene) dumbbell-
shaped molecule.5
Mixtures of 1 and 2 were studied using molar ratios of 200:1,
500:1, and 1000:1. AFM images of the 200:1 mixture of 1 and 2
revealed small aggregates (Figure 2b). The AFM of the 200:1
mixture appears completely different from the control sample of 1
alones1 and 2 interact to form small structures rather than high-
aspect-ratio nanofibers. The 200:1 solution of aggregates has heights
of 5.5 ( 0.7 nm by AFM, comparable to fiber heights of 5.3 ( 0.6
nm. In mixed samples with a higher molar fraction of PA 1 (500:1
or 1000:1), we observe both small micelle-like nanostructures and
longer nanofibers, indicating that there is not enough of template
2 to fully suppress the extensive one-dimensional assembly of 1
(see Supporting Information).
† Department of Chemistry.
The fiber widths from 1 were determined by TEM to be 5.1 (
0.7 nm (see Supporting Information). As in the AFM images, the
TEM images of the 200:1 mixture show micelle-like structures with
diameters of approximately 5.6 ( 0.6 nm. We attribute the
‡ Department of Chemical & Biological Engineering.
§ Departments of Biochemistry and Molecular and Cellular Biology, Neuro-
biology and Physiology, and Radiology.
¶ Department of Materials Science & Engineering and Feinberg School of
Medicine.
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10.1021/ja710749q CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
2, we expect the PA to dominate the spectrum, but we cannot
exclude the possibility of chirooptic activity from the oligo-
(phenylene ethynylene).9 The fluorescence spectrum of 2 remains
unchanged after co-assembly with 1, as we would expect for a
molecule with a small dipole. While it is difficult to prove direct
interaction between 1 and 2 spectroscopically, we believe that the
microscopy and light scattering data provide compelling evidence
for a templated change in the self-assembly behavior.
We conclude that the formation of nonspherical supramolecular
aggregates with controlled dimensions has been realized using a
rigid-rod dumbbell-shaped template that limits the one-dimensional
self-assembly of PA molecules. AFM and TEM data show a
dramatic change in the form of the PA supramolecular aggregate
upon addition of the dumbbell template from high-aspect-ratio
nanofibers to small, nearly monodisperse nanostructures. DLS and
CD confirm this behavior in solution. This general strategy may
provide control over properties of supramolecular aggregates that
are sensitive to dimension and may offer more precise control of
their stoichiometric composition. The latter has potential to control
the delivery of therapeutic molecules in supramolecular form.
Finally, shape control within the molecular template may lead to
greater complexity in the supramolecular nanostructure.
Figure 3. QFDE TEM of 200:1 molar ratio mixture of 1 to 2, revealing
the aspect ratio of the small aggregates. Inset: The length and width are in
agreement with the size expected for the templated assembly. The dark
exteriors in the replica result from the deposition of the platinum.
appearance of these round structures to an end-on view of the
aggregate. These data indicate that the 1‚2 aggregate has a diameter
similar to that of nanofibers formed by PA molecules alone,
suggesting the template molecules can control the length of open,
nonspherical supramolecular structures.
Acknowledgment. The authors thank Northwestern University’s
NIH Biotechnology Training Program (predoctoral fellowship
to S.R.B.), the Department of Homeland Security (predoctoral
fellowship to M.A.G.), the National Science Foundation, the
Department of Energy, and the National Institutes of Health
(5R01EB005866-02 and CA119341-02) for financial support. This
work made use of NUANCE (TEM, AFM), Biological Imaging
Facility (TEM), IMSERC (NMR, MS), and Keck Biophysics
Facility (CD).
To better understand the structure of the 200:1 assembly without
the effects of drying, we prepared additional TEM samples
according to a quick-freeze deep etch (QFDE) protocol. Since the
sample is prepared by subliming ice from a frozen solution, we
expect the small aggregates to appear in a variety of orientations.
Indeed, we observed slightly elongated nanostructures that we can
estimate to have dimensions of ∼8.8 nm diameter and 14.0 nm
length (Figure 3). TEM images are taken from replica prepared by
sputter-coating ∼1.2 nm of platinum onto the sample, so the
measured lengths and widths should be ∼2.4 nm greater than the
actual size. In fact, we observe that the small aggregates display
an aspect ratio consistent with the dimensions of our proposed
mechanism for templated self-assembly (∼5.5 nm × 12.2 nm).
Dynamic light scattering (DLS) was used to characterize the
effect of the dumbbell template on PA assembly in solution. A
change in the self-assembled structures from long fibers to small
aggregates should result in a measurable change in hydrodynamic
radius. DLS measurements on solutions of 1 confirmed the presence
of large structures (>1 µm, see Supporting Information). While 2
was observed to form ribbon-like structures by AFM as a film on
mica (attributed to drying effects), DLS indicates that, in solution,
2 has a hydrodynamic radius of ∼17.4 nm. The mixed sample
consisting of a 200:1 molar ratio gave a size distribution centered
at ∼17.9 nm. We do not expect a significant change in hydrody-
namic radius of the dumbbell molecules with and without the PA,
since the long axis of the assembly is the same as that of the
template. In the mixed samples of 500:1 and 1000:1 (consistent
with the microscopy data), we observe scattering from large
structures that suppresses any signal from smaller species, indicating
the persistence of fibers. Circular dichroism (CD) revealed the
presence of â-sheet structure in solutions of 1 alone and in the
200:1 mixture, confirming that the PA molecules were still highly
ordered in the 1‚2 aggregate. While 2 limits the ability of 1 to form
extended supramolecular structures, the intermolecular basis of PA
self-assembly remains unchanged. At such a low concentration of
Supporting Information Available: Synthetic procedures, char-
acterization of new compounds, additional microscopy, dynamic light
scattering, and spectroscopic data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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